The Amoebas
Protozoa are unicellular organisms and the lowest form of animal life.
In the subkingdom Protozoa, there are three phyla of medical interest
in humans. The phylum Sarcomastigophora, subphylum Sarcodina,
includes the pathogenic and nonpathogenic amebas.
The most important feature that separates amebas from the other
groups of unicellular Protozoa is the means by which they move.
Amebas are equipped with the ability to extend their cytoplasm in the
form of pseudopods, which allows. Them to move within their
environment.
With one exception, there are two morphologic forms in the amebic
life cycle – trophozoites and cysts
Excystation – morphologic conversion from the cyst from into the
trophozoite form, occurs in the ileocecal are of the intestine
Encystation – conversion of trophozoites to cysts
Entamoeba histolytica
Geographical distribution: Cosmopolitan
Habitat
•
Trophozoite – large intestine, liver abscesses, and other
extraintestinal organs
•
Cysts – found in the stools of chronic dysenteric patients
and carriers
Morphology
Cyst
•
size – 12-15 um (1 ½ - 2 RBCs)
•
shape – spherical
•
nuclei – 1-4 nuclei
•
nuclear membrane – thin, regular, and circular lined with
fine chromatin granules internally, and small, compact
central karyosome.
Trophozoite
•
size – 12 – 35um, usually as long as 3 or 4 RBCs
•
shape – elongated form when actively motile and rounded
form when at rest
•
motility – active, progressive, directional amoeboid motility
in fresh warm stool specimen
•
pseudopodia – finger-like, broadly rounded end
•
cytoplasm – well differentiated into ectoplasm and
endoplasm. May contain ingested host’s RBCs in
dysenteric specimens
•
Nucleus – single nucleus, not visible in the motile form but
in iodine stained smear clearly seen.
Mode of Transmission
Infective Stage – tetra-nucleated mature cyst.
Man acquires infection of E. histolytica from ingestion of food or drink
contaminated by infective cyst
Clinical Features and Pathology
May be asymptomatic or exhibit amoebic dysentery or extra-intestinal
amoebiasis in the liver, brain, spleen, lung, etc.
Treatment
Metronidazole (alternatives: tinidazole, ornidazole, and nitazoaxinide)
Entamoeba coli
Geographical distribution: Cosmopolitan
Habitat – both trophozoite and cysts in the large intestine of man
Morphology
Trophozoite
•
Size – 15-50 um (average 25 um), usually bigger than E.
histolytica
•
shape – oval or elongated
•
motility – sluggish, non-progressive, and non-directional
short blunt pseudopodia
•
cytoplasm – ectoplasm and endoplasm not well
differentiated
•
nucleus – single nucleus, visible in the fresh state without
staining. Thick nuclear membrane lined with coarse
chromatin granules and eccentric karyosome.
•
Inclusions: bacteria, yeast, but never RBCs
Cyst
•
Size 12-25um
•
Spherical
Iodamoeba butschlii
Balantidium coli
Giardia lamblia
Different stages of Haemoflagellates
Trypanosoma brucei – gambiense & rhodesiense
Trypanosmoa cruzi